Numerous products are manufactured from a porous sintered body i.e. a material that has a multiple of voids within. These products include filters, catalysts for chemical reactions, electrodes, heat exchangers and so on. When the pores are used to hold lubricant oil or the like, the porous sintered body can be used as a material for bearings.
Methods of manufacturing the porous sintered body include casting method, foaming method, plating method, etc. Another method is space holder method: In this method, powdery sintering compound is mixed with a pore formation material which is a compound that is removed by heat. The mixed compound is molded, then heated to remove the pore formation material thereby making pores, and thereafter the sintering compound is sintered into a porous sintered body.
JP-A 2001-2271101 Gazette for example, discloses a method of making a sintered body by using space holder method.
The patent document discloses a method of making a sintered body which uses a composite including a row material powder, an organic binder, and a pore formation material whose decomposition starting temperature is higher than that of the organic binder. The method includes a step of forming a formed body, a first degreasing step of removing the organic binder thereby obtaining a first degreased body, a second degreasing step of removing the pore formation material thereby obtaining a second degreased body, and a step of sintering the second degreased body thereby obtaining a sintered body.
In the Patent Document 1, it is described that according to the invention therein, it is possible to secure a path for removal of the pore formation material, by prior removal of the organic binder.
According to the method of making a sintered body disclosed in the Patent Document, first, most of the binder is removed in the first degreasing step. Next, the first degreased body is heated further, thereby removing the pore formation material. Thus, at the time when the pore formation material has been removed, there is no binder remaining.
However, the pore formation material generally has a larger particle size than the binder and the sintering compound particles. On the other hand, in a formed body before the degreasing process, the sintering compound particles and the binder are loaded to fill the surrounds of the pore formation material. Thus, in the invention disclosed in the Patent Document, the sintering compound particles after removal of the binder are supported by the pore formation material, around the pore formation material. Then, as the pore formation material is removed under this state, the sintering compound particles around the pore formation material are likely to drop into the space formerly occupied by the pore formation material, or walls that define pores are likely to deform. As a result, the formed body is easily deformed in the degreasing step, and it is difficult to obtain a sintered body of a desired size and shape.
In particular, when making a porous body which has a high rate of porosity, walls provided by the sintering compound particles around the pores tend to be thin, and are likely to deform for example, in the degreasing step, making it difficult to form pores of a desired shape and size, and thus making impossible to obtain a sintered body of a desired porosity. As a result, according to the Patent Document, it is impossible to form a porous sintered body which has a porosity not smaller than 50%.
Currently, porous sintered bodies which offer novel functions are available. These are made from a compound mixed with a functional particle material which works as a catalyst for example. The functional material mixed with the sintering compound is molded and sintered into products. However, according to such a sintered body produced by sintering a sintering compound mixed with a functional particle material, only part of the functional particle material can be on a surface, and have exposure to the pore space. Thus, it has not been possible to make the product as effective as the amount of functional material particles mixed with the compound. Therefore, if the product is to be effective in terms of functionality provided by the functional particle materials mixed therein, a large amount of the functional particle must be added, which makes it impossible to use expensive material such as platinum.
One idea is to form a porous sintered body first and then fill the pores with a functional material. A problem, however, is that only a limited amount of functional material can be filled in the sintered body if the body has a small porosity. Another problem is that functional material in the form of particle is difficult to fill after the body has been sintered and the amount which can be filled is also limited unless the pores are large. It may not be impossible to form pores first, and then fill the material before sintering; however, the conventional porous formed body as disclosed in the Patent Document does not have enough strength after the pores are formed, so filling is an impossible idea.